The overall goal of this proposal is to characterize the mechanical properties of pericardium, infarcted myocardium, and contracting muscle (diaphragm) in terms of their stress-strain relationships obtained during multiaxial stretching. The goal will be sought by addressing the following specific hypotheses: (1) Observed regional in vivo differences in pericardial deformations and contact pressures can be explained by regional differences in mechanical properties. (2) Fibrous membranes adherent to both myocardium (the epicardium) and diaphragm have different properties that the underlying muscle and contribute significantly to the mechanical properties of the overall structure. (3) As transmural myocardial infarctions age they become progressively stiffer and their material symmetry changes from anisotropic to more isotropic. (4) Contracting muscle is less anisotropic and has a more linear stress- strain relation than the same muscle in the passive state. (5) Transverse stiffness in an indentation test is predictably related to the relative dimensions of the punch, depth of indentation, thickness and intrinsic properties of the material. The approach is to first determine the material symmetry by using theoretical, structural or experimental considerations. Specific functional forms of the constitutive law are then determined by employing biaxial experimental data appropriate to that form of symmetry rather that being selected ad hoc as has been done in the past. Finally, parameter values of each constitutive relation are obtained from equibiaxial tests in which undeformed orientations are preserved. This method should produce more reliable and generally applicable results that methods now in use. The results should provide better insight into many aspects of cardiac mechanics including the interaction between heart and pericardium, the mechanisms underlying dysfunction of infarcted hearts, and the function of normal tissue.